The present invention relates to secondary agglomerates of magnetic metal particles for magnetic recording and a process for producing the same, and more particularly, to secondary agglomerates of magnetic metal particles for magnetic recording, which exhibit not only excellent handing property due to high storage efficiency, high transport efficiency and good flowability thereof, but also excellent kneading property when kneaded with various binder resins and organic solvents in a kneader, and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, so as to more improve a surface smoothness and squareness of a magnetic coating film obtained therefrom, and a process for producing such secondary agglomerates of magnetic metal particles for magnetic recording.
In recent years, recording-time prolongation, miniaturization and lightening of audio, video or computer magnetic recording and reproducing apparatuses for various magnetic recording media such as digital audio tapes (DAT) for people""s livelihood use, 8-mm video tapes, Hi-8 tapes, VTR tapes for business use, computer tapes or discs thereof have proceeded more rapidly. In particular, VTRs (video tape recorders) are now widespread, so that there have been intensely developed VTRs aiming at the transfer of analog recording types into digital ones in addition to the above recording-time prolongation, miniaturization and lightening thereof. On the other hand, with such recent tendencies, the magnetic recording media have been required to have high image quality and high output characteristics, especially high frequency characteristics. To meet these requirements, it is necessary to reduce noise due to the magnetic recording media themselves and enhance residual magnetic flux density, coercive force, dispersibility, filling property and tape-surface smoothness thereof. Therefore, it ha been further required to improve S/N ratio of the magnetic recording media.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles have been noticed because such particles can show a higher coercive force and a larger saturation magnetization as compared to those of conventional magnetic iron oxide particles, and have been already used as magnetic particles for magnetic recording media such as DAT, 8-mm video tapes, Hi-8 tapes, video tapes for business use, computer tapes or discs. The magnetic metal particles containing iron as a main component conventionally used in DAT, 8-mm video tapes, Hi-8 tapes or the like have been required to be further improved in output characteristics and weather resistance. In addition, the magnetic metal particles must fulfill applicability to existing format and good economy at the same time. Therefore, it has been strongly required to provide magnetic metal particles capable of satisfying the above requirements while minimizing amounts of various metals added thereto.
Various properties of coating-type magnetic recording media are detailed below.
In order to obtain high image quality in video magnetic recording media, it has been required to enhance S/N ratio and video frequency characteristics thereof. For this reason, it is important to improve a surface smoothness of the magnetic recording media. For improving the surface smoothness, it is also required to improve a dispersibility of magnetic particles in coating composition as well as orientation and filling properties thereof in coating film. In addition, in order to enhance the video frequency characteristics, the magnetic recording media have been required to exhibit not only a high coercive force and a large residual magnetic flux density, but also an excellent S.F.D. (Switching Field Distribution), i.e., a small coercive force distribution.
As known in the arts, the magnetic metal particles are produced by granulating as a starting material goethite particles, hematite particles obtained by heat-dehydrating the goethite particles, or particles obtained by incorporating metal elements other than iron into the above goethite or hematite particles, to form a granulated product having an appropriate size; and then heat-reducing the resultant granulated product. In this case, it is important to appropriately control the shape and size of the goethite particles as a starting material, and prevent occurrence of heat fusion between particles as well as deformation and shape breakage of each particle upon heat-treatments such as heat-dehydration and heat-reduction.
In general, the granulated product of magnetic metal particles is directly charged into a kneader and kneaded therein with various binder resins and organic solvents.
More specifically, in the production of coating-type magnetic recording media, after the granulated product of magnetic metal particles is kneaded with various binder resins and organic solvents in the kneader, the obtained kneaded material is mixed and diluted with an additional amount of the organic solvent and the obtained magnetic coating composition is coated on a non-magnetic substrate. As described above, since the dispersibility of the magnetic metal particles influences the surface smoothness of the magnetic coating film obtained therefrom, the poorer the dispersibility of the magnetic metal particles, the lower the squareness of the magnetic coating film. Therefore, it has been strongly required that the magnetic metal particles have an excellent dispersibility, and the granulated product of magnetic metal particles is readily deaggregated into the magnetic metal particles as primary particles.
On the other hand, it is preferred that the granulated product of magnetic metal particles has a bulk density as large as possible. This is because when the granulated product is filled in a container for temporary storage in warehouse or transport thereof, the size of the container used therefor can be reduced, thereby saving the storage space and reducing the transport cost. However, in general, particles having a large bulk density exhibit a deteriorated flowability, thereby causing such a clogging or bridging phenomenon that the discharge of the particles from an outlet of storage or transport container or receptacle is completely disturbed or frequently interrupted even though the outlet diameter is far larger than the size of the particles, i.e., causing considerable deterioration in handling property of the particles. Therefore, it has been strongly required to provide granulated product having an appropriate flowability in addition to other good properties.
It is generally known that the flowability of the granulated product varies depending upon particle diameter, particle density, particle shape and surface property thereof. Therefore, the present inventors"" attention has been paid to the particle diameter and particle density.
As conventional techniques for improving magnetic properties, dispersibility, flowability, keeping properties or the like of magnetic particles for magnetic recording media, there are known those described in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-275028(1987), 63-88807(1988) and 3-276423(1991), Japanese Patent Publication (KOKOKU) Nos. 1-52442(1989), 4-70363(1992) and 7-62900(1995), Japanese Patent Application Laid-Open (KOKAI) No. 8-172005(1996) or the like.
At present, it has been strongly required to provide granulated product of magnetic metal particles which can exhibit not only excellent kneading property when kneaded with various binder resins and organic solvents and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, but also high storage efficiency and high transport efficiency as well as good flowability. However, such granulated product of magnetic metal particles cannot be obtained yet.
Namely, in Japanese Patent Application Laid-Open (KOKAI) No. 62-275028(1987), although there is described the method of producing cobalt-containing spherical ferromagnetic iron oxide particles having a particle diameter of 5 to 200 xcexcm by spray-drying for the purpose of obtaining magnetic particles having an excellent dispersibility, it is difficult to directly apply this method to the production of granulated product of magnetic metal particles. This is because the material to be treated is cobalt-containing ferromagnetic iron oxide quite different from the granulated product, and the method is concerned with drying (spray-drying) method which is inherently required when such particles are recovered from water slurry after adhering Co thereto. When the magnetic metal particles are treated in the form of water slurry, there tend to arise problems such as deteriorated magnetic properties due to oxidation in the aqueous system or upon drying.
In Japanese Patent Application Laid-Open (KOKAI) No. 63-88807(1988), although there is described the method of producing magnetic particles by finely pulverizing baked particles, granulating the finely pulverized particles using water as a binder, drying and then reducing the granulated product for the purpose of increasing the density of the magnetic particles and shortening the time required for dispersing the particles in a magnetic coating composition simultaneously, the diameter of the granulated product optimum for kneading upon the production of magnetic coating composition and flowability of these particles are not specified at all. Therefore, this method fails to provide a granulated product of magnetic metal particles having sufficient dispersibility and handling property in the magnetic coating composition.
In Japanese Patent Application Laid-Open (KOKAI) No. 3-276423(1991), although there is described the method of producing a magnetic recording medium using ferromagnetic particles capable of being introduced into a kneader with a high flowability and an accurate quantity, the diameter of the granulated product is not specified at all. Therefore, this method also fails to sufficiently improve kneading property and dispersibility of ferromagnetic particles.
In Japanese Patent Publication (KOKOKU) No. 1-52442(1989), there is described the method of producing magnetic metal particles by granulating and shaping iron oxide hydroxide or iron oxide into massive granulated product having a diameter of 0.5 to 30 mm, supplying the granulated product into a tubular reducing furnace, and heat-reducing the granulated product while passing a reducing gas therethrough in order to uniformly conduct the reduction reaction and simultaneously prevent the splash of particles. In this method, kneading property, dispersibility, flowability, storage efficiency and transport efficiency of the granulated product of magnetic metal particles are not specified at all.
In Japanese Patent Publication (KOKOKU) No. 4-70363(1992), there is described the method of forming a fine oxide film and stabilizing magnetic properties thereof by shaping iron oxide hydroxide particles or iron oxide particles into pellets, heat-reducing the pellets to obtain pellets of metal particles, oxidizing the pellets to form an oxide film on the surface of each metal particle, and pulverizing the resultant pellets into particles having a size before the pelletization. In this method, in view of such a fact that the granulated product of magnetic metal particles is pulverized into the size before pelletization, the storage efficiency and transport efficiency of the granulated product are not considered at all. Therefore, this method also fails to provide the granulated product of magnetic metal particles having sufficient dispersibility and flowability.
In Japanese Patent Publication (KOKOKU) No. 7-62900(1995), there is described the method of compacting fine ferromagnetic metal particles by sand mill, etc., in order to deaggregate an associated product of the fine ferromagnetic metal particles (including aggregates, agglomerates and softly associated product) and increase the content of primary particles in the fine ferromagnetic metal particles. In this method, although the associated product is strongly deaggregated into primary particles by applying a predetermined linear pressure thereto, the flowability thereof is not specified at all. As described in Comparative Example 3 hereinafter, such a method fails to provide a granulated product having a sufficient flowability. Also, in this method, the diameter of the granulated product of magnetic metal particles is not specified at all.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 8-172005(1996), although there are described metal iron particles for magnetic recording having a bulk density of 0.55 to 1.0 g/ml, the diameter of the granulated product optimum for kneading and the flowability thereof are not taught nor suggested.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by granulating and shaping a starting material such as goethite particles having an average major axial diameter of 0.05 to 0.40 xcexcm as primary particles or hematite particles obtained by heat-dehydrating the goethite particles; heat-reducing the resultant granulated product to obtain a granulated product of magnetic metal particles; and pulverizing the obtained granulated product of magnetic metal particles using an apparatus having a crushing function for deaggregating the granulated product of magnetic metal particles by a rotor and a sizing function for forcibly passing the crushed product through a screen,
the thus obtained secondary agglomerates of magnetic metal particles can exhibit not only a high storage efficiency, a high transport efficiency and an excellent handling property due to a good flowability thereof, but also excellent kneading property when kneaded with various binder resins and organic solvents in a kneader and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, and as a result, the surface smoothness and squareness of a magnetic coating film obtained therefrom are further improved. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide secondary agglomerates of magnetic metal particles which can exhibit not only a high storage efficiency, a high transport efficiency and a good flowability thereof, but also excellent kneading property when kneaded with various binder resins and organic solvents in a kneader and excellent dilution-dispersibility when diluted with an additional amount of the organic solvent, upon the production of coating-type magnetic recording media, and as result, the surface smoothness and squareness of a magnetic coating film obtained therefrom are further improved.
Another object of the present invention is to provide a process for producing the above secondary agglomerates of magnetic metal particles in an efficient and industrially advantageous manner.
To accomplish the aims, in a first aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0.
In a second aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm, a repose angle of 38 to 45xc2x0, an average particle diameter of not more than 53 xcexcm in an amount of not more than 30% by weight, a bulk density of 0.35 to 0.65 g/ml, a tap density of 0.39 to 0.75 g/ml and a compaction percentage of 10 to 15%.
In a third aspect of the present invention, there is provided a process for producing the secondary agglomerates of magnetic metal particles as defined in the first aspect, which process comprises:
granulating and shaping goethite particles comprising primary particles having an average major axial diameter of 0.05 to 0.40 xcexcm or hematite particles obtained by heat-dehydrating the goethite particles as a starting material;
heat-reducing the resultant granulated product of goethite or hematite particles to obtain a granulated product of magnetic metal particles; and
deaggregating the obtained granulated product of magnetic metal particles using an apparatus having a crushing function for deaggregating the granulated product of magnetic metal particles by a rotor and a sizing function for forcibly passing the crushed particles through a screen.
In a fourth aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0, and
said primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, an aspect ratio of 4:1 to 13:1 and a BET specific surface area of 35 to 65 m2/g.
In a fifth aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0, and
said primary particles having an average major axial diameter of 0.05 to 0.15 xcexcm, an aspect ratio of 5:1 to 9:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.30, a crystallite size D110 of 130 to 160 xc3x85, a Co content of from 0.5 to less than 6 atm % based on whole Fe, an Al content of from more than 10 to less than 20 atm % based on whole Fe, a rare earth content of 1.5 to 5 atm % based on whole Fe, an atomic ratio of Al to Co of from more than 2 to 4, a coercive force of 111.4 to 143.2 kA/m, an oxidation stability (xcex94"sgr"s) of saturation magnetization of not more than 10%, and an ignition temperature of not less than 130xc2x0 C.
In a sixth aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0, and
said primary particles having an average major axial diameter (L) of 0.05 to 0.15 xcexcm; a coercive force of 111.4 to 143.2 kA/m; a Co content of from 0.5 to less than 5 atm % based on whole Fe; a crystallite size of from 150 to less than 170 xc3x85; a specific surface area (S) represented by the formula:
S less than xe2x88x92160xc3x97L+65; 
an oxidation stability (xcex94"sgr"s) of saturation magnetization of not more than 5%; and an ignition temperature of not less than 140xc2x0 C.
In a seventh aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0, and
said primary particles having a Co content of from 0.5 to less than 10 atm % based on whole Fe, an Al content of from 5 to 10 atm % based on whole Fe, a rare earth content of from 1 to 5 atm % based on whole Fe, an atomic ratio of Al to rare earth element of 1.5 to 5, calculated as atm % of the respective elements based on Fe, an average major axial length of 0.05 to 0.25 xcexcm, a size distribution (standard deviation/major axial length) of not more than 0.26, an average minor axial length of 0.015 to 0.025 xcexcm, an average aspect ratio of 5:1 to 9:1, a specific surface area of 30 to 60 m2/g, an ignition temperature of not less than 145xc2x0 C., an oxidation stability (xcex94"sgr"s) of not more than 6%, and a coercive force of 103.5 to 143.2 kA/m.
In an eighth aspect of the present invention, there are provided secondary agglomerates of magnetic metal particles comprising magnetic metal primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm and a repose angle of 38 to 45xc2x0, and
said primary particles having a Co content of from 0.5 to less than 10 atm % based on whole Fe, an Al content of 5 to 10 atm % based on whole Fe, a rare earth content of 1 to 5 atm % based on whole Fe, an atomic ratio of Al to rare earth element of 1.5 to 5, calculated as atm % of the respective elements based on Fe, an average major axial length of 0.15 to 0.25 xcexcm, a size distribution (standard deviation/major axial length) of not more than 0.30, an average minor axial length of 0.015 to 0.025 xcexcm, an average aspect ratio of 5:1 to 9:1, a specific surface area of 30 to 60 m2/g, an ignition temperature of not less than 135xc2x0 C., an oxidation stability (xcex94"sgr"s) of not more than 10%, and a coercive force of 103.5 to 143.2 kA/m.
In a ninth aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and a magnetic recording layer formed on the non-magnetic substrate comprising a binder resin and magnetic metal particles containing iron as a main component which are derived from secondary agglomerates of magnetic metal particles containing primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm a repose angle of 38 to 45xc2x0.
In a tenth aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and a magnetic recording layer formed on the non-magnetic substrate comprising a binder resin and magnetic metal particles containing iron as a main component which are derived from secondary agglomerates of magnetic metal particles containing primary particles having an average major axial diameter of 0.05 to 0.25 xcexcm, said secondary agglomerates having an average particle diameter of 300 to 800 xcexcm, an upper limit of particle diameter of 2,000 xcexcm a repose angle of 38 to 45xc2x0, having a coercive force value of 111.4 to 143.2 kA/m, and when the magnetic coating film is oriented by applying a magnetic field of 397.9 kA/m thereto, a squareness (Br/Bm) of not less than 0.84, an orientation property (OR) of usually not less than 2.8, a coercive force distribution (Switching Field Distribution) of not more than 0.53 and an oxidation stability (xcex94Bm) of not more than 8.0%.